Method and apparatus for code assignment in a spread spectrum wireless communication system

ABSTRACT

Method and apparatus for code assignments in a spread-spectrum wireless communication system incorporating a Large Area Synchronized-Code Division Multiple Access (LAS-CDMA) protocol. The method determines a set of LS codes based on an interference free window size. An arborescence structure provides the correspondence between the interference free window size and the set of LS codes. Subsets of the LS codes are formed having null cross-correlation within the interference free window. The subsets are assigned to neighboring cells in the system to reduce interference between neighbors. In one embodiment, a controller determines the subsets and makes the assignments to cells within the system.

CLAIM OF PRIORITY UNDER 35 U.S.C. §120

[0001] The present application for patent is a Continuation and claimspriority to patent application Ser. No. 09/737,893 entitled “METHOD ANDAPPARATUS FOR code assignment in a spread spectrum WIRELESScommunication system,” filed Dec. 15, 2000, now allowed, and assigned tothe assignee hereof and hereby expressly incorporated by referenceherein.

BACKGROUND

[0002] 1. Field

[0003] The present invention relates to wireless data communication.More particularly, the present invention relates to a novel and improvedmethod and apparatus for code assignment in a spread-spectrum wirelesscommunication system.

[0004] 2. Background

[0005] In a spread spectrum wireless communication system, a basestation communicates with multiple mobile users. In one system, a CodeDivision Multiple Access (CDMA) system, such as specified in the“TIA/EIA/IS-95 Mobile Station-Base Station Compatibility Standard forDual-Mode Wideband Spread Spectrum Cellular System,” hereinafterreferred to as “the IS-95 standard,” or the “TIA/EIA/IS-2000 Standardsfor cdma2000 Spread Spectrum Systems,” hereinafter referred to as “thecdma2000 standard,” codes are applied to the data and controlinformation. The codes identify the target recipient as well as thesender. Operation of a CDMA system is described in U.S. Pat. No.4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATIONSYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,” and also in U.S. Pat.No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN ACDMA CELLULAR TELEPHONE SYSTEM,” both assigned to the assignee of thepresent application for patent and hereby expressly incorporated byreference. The forward link from base station to mobile users assigns aunique Walsh code to each mobile with which it transmits, wherein theWalsh code identifies the mobile. Similarly, the reverse link from themobile user to the base station uses a Pseudorandom Noise (PN) code forchannelization.

[0006] Alternate spread spectrum systems incorporate a variety of codesfor the two-way identification. A problem exists as codes may be reusedin neighboring cells and/or sectors, creating interference for adjacentneighbors. Therefore, a method is needed to provide the two-wayidentification of wireless communications while minimizing and/orreducing the interference experienced by neighboring cells and/orsectors. Similarly, there is a need for a code assignment method thatachieves that reduces neighbor interference.

SUMMARY

[0007] The disclosed embodiments provide a novel and improved method forcode assignment in a spread spectrum wireless communication system. Inone aspect, in a wireless communication system adapted for Large AreaSynchronized-Code Division Multiple Access (LAS-CDMA) transmissionshaving a plurality of LS codes, a method includes determining a size ofan Interference Free Window (IFW), calculating a plurality of subsetsfrom the plurality of LS codes, each subset comprising LS codes as afunction of the IFW, assigning a first of the plurality of subsets to afirst portion of the system, and assigning a second of the plurality ofsubsets to a second portion of the system.

[0008] In another aspect, a wireless communication system incorporatingan LAS-CDMA protocol includes a first cell, the first cell beingassigned a first subset of LS codes; and a second cell, the second cellbeing assigned a second subset of LS codes, wherein the first and secondsubsets have a null cross-correlation within a predeterminedinterference free window. Note that the LS codes within the first andsecond subsets also have a null cross-correlation with a predeterminedIFW that is typically wider than that between cells.

[0009] In another aspect, in a Large Area Synchronized-Code DivisionMultiple Access wireless communication system, a method includestransmitting a first communication within a first cell, the firstcommunication identifying at least one mobile station within the firstcell by a first LS code within a first subset of LS codes; andtransmitting a second communication within a second cell, the secondcommunication identifying at least one mobile station within the secondcell by a second LS code within a second subset of LS codes, wherein across-correlation of the first and second subsets is null within aninterference free window.

[0010] In another aspect, a controller in a Large Area Synchronized-CodeDivision Multiple Access wireless communication system includes a firstset of instructions for determining an interference free window size forcommunications between neighboring cells; and a second set ofinstructions for determining at least two sets of LS codes based on theinterference free window size, a first set for identifying mobilestations in a first cell and a second set for identifying mobilestations in a neighboring cell, wherein the two sets of LS codes have anull cross-correlation within an interference free window. This is alsotrue of the LS codes within each of the subsets with respect to eachother.

[0011] According to still another aspect, in a Large AreaSynchronized-Code Division Multiple Access wireless communicationsystem, the system having a first cell and a first cell neighborhood, acomputer data signal is embodied on a carrier wave, the signal includinga first portion comprising information for transmission to a firstmobile user in a first cell; and a second portion comprising a first LScode corresponding to the first mobile user, wherein the first LS codeis part of a first set of LS codes exclusively assigned to the firstcell within the first cell neighborhood.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The features, objects, and advantages of the presently disclosedmethod and apparatus will become more apparent from the detaileddescription set forth below when taken in conjunction with the drawingsin which like reference characters identify correspondingly throughoutand wherein:

[0013]FIG. 1 illustrates in block diagram form a wireless communicationsystem according to one embodiment;

[0014]FIG. 2 illustrates in block diagram form a protocol fortransmitting data in a wireless system according to one embodiment;

[0015]FIG. 3 illustrates in tabular form and timing diagram form pulsecode position assignments for LA codes in a Large Area Synchronized(LAS) code system according to one embodiment;

[0016]FIG. 4 illustrates various Interference Free Windows (IFWs) in aLAS code system according to one embodiment;

[0017]FIG. 5 illustrates a permutation table of LA codes in a LAS codesystem according to one embodiment;

[0018]FIG. 6 illustrates an arborescence structure for determining acalculation of LS code length as a function of Interference Free Window(IFW) size;

[0019]FIG. 7 illustrates transmission protocols for mobile units in aLAS code system according to one embodiment; and

[0020]FIGS. 8A and 8B illustrate in block diagram form various codeassignment schedules for a LAS code system according to alternateembodiments.

DETAILED DESCRIPTION

[0021] In many spread-spectrum communication systems codes are appliedto transmission signals for channelization. Often a first type of codeis applied to the data signals to identify the designated mobile user,and a second type of code is applied that is specific to the basestation. A CDMA system, for example, has a large number of codesavailable for spreading the data and control information transmitted ina wireless system, allowing one base station to communicate withmultiple mobile users. The CDMA codes include a Walsh code assigned toeach mobile user and a Pseudorandom Noise (PN) code specific to the basestation. Both codes are applied to data signals transmitted by the basestation. A set of Walsh codes is a set of orthogonal binary sequences,wherein the cross-correlation over time is zero. The Walsh codes aregenerated using a Hadamard matrix, wherein recursion allows expansion ofa base code, or seed, into lengthier codes, thus increasing the size ofthe Walsh code set.

[0022] However, in contrast to Walsh codes, PN codes do not requiresynchronization such as used to implement Walsh codes. Rather, PN codesmay be generated by linear feedback shift registers, wherein binary bitsare shifted through the different stages of the register. Output bitsfrom the last stage form the PN codes. PN codes have an auto-correlationcharacteristic that allows codes to align at the receiver. Thecombination of Walsh and PN codes allows identification of the basestation and mobile station for wireless transmissions. Other systems mayincorporate other codes and/or combinations of Walsh, PN, and othercodes to identify the mobile user and the base station.

[0023] Large Area Synchronized-Code Division Multiple Access (LAS-CDMA)Systems

[0024] In another type of spread spectrum wireless communication system,referred to as a Large Area Synchronized-Code Division Multiple Access(LAS-CDMA) system, specially designed codes, referred to as “LS” and“LA” codes, are used to spread the signals for transmission. LAS-CDMA isa technique for spread-spectrum and time-division multiple access thatcreates channels through the use of time division and orthogonal codes.One LAS-CDMA system is described in the “Physical Layer Specificationfor LAS2000,” a candidate proposal to the China WirelessTelecommunication Standard (CWTS).

[0025] In a LAS-CDMA system, communications are transmitted in frames,wherein a fixed number of sub-frames are included in each frame. Eachsub-frame is then segmented into a predetermined number of time slots.The LS and LA codes are used for channelization, as in other CDMAsystems, wherein the LS and LA codes are designed to have a small orzero cross-correlation over time.

[0026] A first type of code, the LS code, is applied to the data orsymbols in each time slot. The LS code identifies the mobile user thatis the target of the transmission. Within a given cell and/or sector,different LS codes are applied to each mobile user. In preparing eachtransmission, the appropriate LS code is modulated by a symbol, asdescribed hereinbelow, to form a symbol modulated LS code.

[0027] A second type of code, the LA code, identifies the cell and/orsector, i.e., the base station operating within the area. Unlike the LScode that is applied to each time slot, the LA code is applied to theentire sub-frame, and defines the time allotted to each segment of thesub-frame. Application of a given LA code to the entire sub-frameresults in various sized time gaps between modulated symbols. Thecombination and order of the gap sizes serves as an identification ofthe cell and/or sector.

[0028] An exemplary embodiment of a LAS-CDMA system is illustrated inFIG. 1. The system 10 is a LAS-CDMA system including a plurality ofcells 20, 30, 40, 50, 60, 70, each having a base station 22, 32, 42, 52,62, 72, respectively, for communication with mobile stations 24, 34, 44,54, 64, 74, within the system 10. A forward channel is used fortransmission of data from base stations 22, 32, 42, 52, 62, 72, tomobile stations 24, 34, 44, 54, 64, 74, within system 10. Each mobilestation 24, 34, 44, 54, 64, 74, uses a reverse channel for transmissionof data to at least one of base stations 22, 32, 42, 52, 62, 72. Inother LAS-CDMA systems, the cells may be as configured in FIG. 1, or maybe positioned in an alternate manner, wherein each base station isassociated with at least one cell and/or sector.

[0029] As illustrated in FIG. 1, a first portion of the system 10 isreferred to as cell 20. Other portions of the system 10 that are locatedin close proximity to cell 20 make up a neighborhood of cell 20. Insystem 10, the illustrated portion of the neighborhood of cell 20includes cells 30, 50, and 60. Other portions within the neighborhood ofcell 20 are not shown, but may be located anywhere in close proximity tocell 20. In one embodiment, a cell neighborhood includes those cellsthat border the cell geographically. In alternate embodiments, theneighboring criteria that determines which portions are included in aneighborhood may be made according to the interference experiencedbetween cells, or some other criteria that relates to the interaction ofwireless transmissions within cells.

[0030] Within system 10, signals are prepared for transmission byformatting the signals into frames of a predetermined protocol. FIG. 2illustrates one embodiment of a forward channel protocol 100 used insystem 10 for transmitting data and control information in 20 ms frames,wherein each frame 102 includes a header field 104 and multiplesub-frames 106. The header field 104 designates pilot bursts and controlinformation for the sync word and the sync sub-channel. The header field104 is followed by a sequence of nine sub-frames 106, numbered 0, 1, 2,. . . 8. The sub-frames 106 are evenly spaced within the frame 102. Notethat the header field 104 is made up of a first number of chips and eachof the sub-frames 106 is made up of a second number of chips, whereinthe second number is not necessarily equal to the first number. As usedherein, a chip is defined as the sample in time.

[0031] As illustrated in FIG. 2, each sub-frame 106 includes apredetermined number of time slots 108, wherein each time slot 108corresponds to an LS code modulated by a symbol. In the presentembodiment, each sub-frame 106 includes 17 time slots, labeled 0, 1, 2,. . . 16. Each time slot 108 includes a modulated LS code and asubsequent gap 110 of variable size defined by the LA code. Note thatthe LS code is assigned to the mobile user, while the LA code isassigned to the base station. The modulated LS code is composed of apair of complementary components, identified as “C” and “S” componentsas discussed in detail hereinbelow. Each C component is 64 chips and ispreceded by a 4 chip gap. Each S component is 64 chips and is precededby a 4 chip gap. Therefore, each modulated LS code consumes 136 chips:(4+64+4+64). The minimum size of any time slot 108 is, therefore, 136chips.

[0032] In the present embodiment, the time slots 108 are not assigned acommon chip length, but rather each time slot 108 within a sub-frame 106is assigned a unique chip length. The chip length of each time slot 108is determined by the gap 110 appended to the modulated LS code, whereinthe sizes of the gaps 110 are not uniform over the entire sub-frame 106.The sizes of gaps 110 are determined by the assigned LA code. The LAcode defines the size of each gap 110 within the time slot 108. The LAcode is effectively a pattern that is applied to the entire sub-frame106. LA codes are described in detail hereinbelow.

[0033] In one embodiment, the frame 102 is composed of 23,031 chips. Theheader field 104 is assigned 1545 chips, and each sub-frame 106 isassigned 2559 chips. As each sub-frame 106 contains 17 time slots 108,and each time slot 108 includes an LS code of 136 bits and a variablesize gap 110, each time slot 108 is, therefore, at least 136 chips long,sufficient for the LS code. Note that if a time slot 108 is equal to 136chips, there is no gap 110.

[0034] LA Codes

[0035] The gaps 110 are determined by the LA code of the cell and/orsector. Specifically, a LA code is a code used to determine theboundaries of time slots within a sub-frame and to identify a celland/or sector. LA codes may be designated by the following parameters,(N, K0, K):

[0036] (i) “N” is the number of pulses;

[0037] (ii) “K0” is the minimum pulse interval; and

[0038] (iii) “K” is the total LA code length in chips.

[0039] A pulse is a function with unit energy and infinitesimalduration. Each pulse identifies a time boundary for a time slot 108. Theminimum pulse interval is determined by the size of the LS code. In thepresent embodiment, the LS code consumes 136 chips, and therefore theminimum pulse interval is 136 chips. Alternate embodiments may implementLS codes with different lengths and hence different minimum pulseintervals.

[0040] As the LA code defines the timing boundaries over the entiresub-frame 106, the total LA code length refers to the length of thesub-frame 106, including all time slots 108. In the present embodiment,the total LA code length is 2559 chips. Note that the LA code is aparsing scheme that covers the entire sub-frame 106; and therefore, thetotal LA code length is defined as the length of the sub-frame 106, andnot the length of the LA code that lists the time intervals assigned toeach time slot 108 within the sub-frame 106. With respect to FIG. 2,while one LS code is applied to each time slot 108, one LA code isapplied to the entire sub-frame 106.

[0041] As illustrated in FIG. 2, each sub-frame 106 is 2559 chips,wherein a chip is defined as the sample after spreading. The total LAcode length, K, is given as the length of one sub-frame 106. Eachsub-frame 106 is segmented into a predetermined number of segments, eachsegment allocated sufficient chips to accommodate one LS code. Thenumber of pulses, N, corresponds to the number of segments aftersegmentation. For example, as illustrated in FIG. 2, each sub-frame 106is segmented into 17 segments, i.e., N=17. The minimum pulse interval,K0, is measured in chips and is constrained to a length of at leastequal to one LS code, i.e., 136 chips. While K0 defines the minimumsegment length of a time slot 108, not all time slots 108 are equal tothe minimum segment length, and perhaps no time slot 108 is equal to theminimum segment length. Each time slot 108 is assigned a segment length.When the segment length of the time slot 108 is greater than 136 chips,the difference is made up by gap 110 subsequent to the S component ofthe modulated LS code.

[0042] In the present embodiment, LA codes are given having parameters(17,136,2559), i.e., N=17; K₀=136; and K=2559. An exemplary LA code isillustrated in FIG. 3. Seventeen pulse intervals are assigned indexvalues from 0 through 16, each corresponding to one of the 17 time slots108 of FIG. 2. Each pulse interval has an associated chip length andpulse position in time with respect to the other pulses. The pulsepositions in time for each index value are illustrated below the LAcode. The corresponding pulse positions for the LA code illustrate thechip beginning of each time slot 108. Within the LA code, from the left,the first pulse interval corresponds to a first time slot 108 of FIG. 2(labeled “0”). The first pulse interval is equal to 136 chips, or theminimum pulse interval. Therefore, the first time slot 108 has a gap 110equal to zero. Continuing with the LA code, the next pulse interval isassociated with the second time slot 108, labeled “1.” This interval is138 chips, and therefore the corresponding pulse occurs at(136+138)=274. The second time slot 108 has a gap 110 equal to twochips. Successive pulse intervals within the LA code define the timingboundaries of the time slots 108 within sub-frame 106, wherein the finalpulse occurs at the time corresponding to the 2559th chip. Within the LAcode of the illustrated embodiment, successive pulse intervals are twochips greater than previous pulse intervals, until the end of the LAcode. Alternate embodiments may implement an alternate assignment ofpulse intervals. The combination and order of pulse intervals creates apattern. The pattern is then permuted to create multiple patterns,providing distinct LA codes applicable to multiple cells and/or sectors.

[0043]FIG. 4 illustrates an exemplary permutation table for 16permutations of the primary LA codes of FIG. 3. Each permutation definesa combination of interval index assignments of the primary code, whereineach permutation alters the arrangement and/or order of intervals. TheLA code permutations identify cell and/or sector of the base station. LAcodes are used in LAS-CDMA to create separate multiple accesstransmission channels on a same RF carrier. The channels may be used bydifferent cells and/or sectors.

[0044] LS Codes

[0045] While LA codes are used to identify cell and/or sector, LS codesare used in LAS-CDMA to spread the transmitted signal and createmultiple code divided transmission channels. As illustrated in FIG. 2,an LS code is a complementary orthogonal code pair used to spreadsymbols. Each occurrence of an LS code consists of a pair of codes ofequal length: a C component and a S component. The C and S componentsare separated by gaps. For each LS code, constituent C and S componentsare designed such that the cross-correlation function of the LS codes iszero within a certain duration.

[0046] In one embodiment, the C and S components are generated frominitial seeds. Example seed pairs may be given as follows:

[0047] (C1; S1)=(++; +−)

[0048] (C2; S2)=(−+; −−),

[0049] wherein a “−” indicates a low logic level bit and a “+” indicatesa high logic level bit. In one embodiment, a seed set contains two suchseed pairs. The following rule is used to generate double length codesfrom a seed pair: (C1   C2; S1   S2) (C1 −C2; S1 −S2) (C2   C1; S2   S1)(C2 −C1; S2 −S1),

[0050] wherein a negative indicates the binary complement of theoriginal. The rule may be used to generate continuously longer codes.

[0051] The present embodiment generates subsets of LS codes within eachset of LS codes having a given LS code length. The subsets are designedsuch that the cross-correlation function between any two LS codes in thesubset is zero. The range of offsets within which the zero valuedcross-correlation is maintained is referred to as the Interference FreeWindow (IFW). The range or interval of the IFW is represented as [−d,+d], wherein “d” represents the offset distance from the origin, andwherein the origin is the position where the two LS codes are exactlyaligned. The range is described over the offset origin, wherein theoffset origin refers to no offset between the codes. Thecross-correlation properties of the LS codes allow for Multiple AccessInterference (MAI) rejection and/or reduction.

[0052] Another characteristic of the LS codes is that theauto-correlation is null within an interval centered around the offsetorigin, but not at the origin. The auto-correlation functions aregenerally defined for a sequence x, as:${{R_{x}(i)} = {\sum\limits_{j = 0}^{J - 1}\quad {x_{j}x_{j - i}}}},$

[0053] wherein J is the number of elements in each occurrence of thesequence or code, x is a code element, and j is the code element index.For each successive shift i, the auto-correlation function calculatesthe summation of the product of x_(j) and its shifted version x_(j-i).The auto-correlation property of the LS codes allows rejection and/orreduction of the multipath-induced Inter-Symbol Interference (ISI).

[0054] The number of LS codes of a given LS code length that satisfy thecross-correlation and auto-correlation properties corresponds to a givenIFW size. In other words, the number of LS codes per subset may bedetermined by the IFW size and the LS code length. For a fixed LS codelength, the width of the IFW increases, the number of LS codessatisfying these properties decreases.

[0055]FIG. 5 illustrates several IFWs with respect to offset intervals.The IFW defines the offset between the two codes, wherein thecross-correlation between the two codes is null. The distance from theorigin corresponds to the shift from the origin, wherein the codes areshifted in time with respect to each other. As illustrated, forIFW=[0,0], the window covers the origin. As the window increases, thenumber of LS codes available decreases.

[0056] In one embodiment, LS codes are defined according to anarborescent structure, such as illustrated in FIG. 6. The structurebegins with a code length of two bits, wherein each of the C and Scomponents are one bit. Progressing down the structure, four LS codesare provided with two bit components, followed by eight LS codes of fourbit components, and finally through to 128 LS codes of 64 bitcomponents. For a 128 bit LS code, the desired IFW determines the numberof codes available to form a LS code subsets. For example, anIFW=[−7,+7] the subset includes 16 codes. As the size of the windowdecreases, the number of codes available for the subset increases. ForIFW=[−3,+3] the subset includes 32 codes; for IFW=[−1,+1] the subsetincludes 64 codes; and for IFW=[0,0] the subset includes all 128 codes.Note that IFW=[0,0] indicates that there is effectively no IFW.

[0057] The following table provides an example for LS code length of 128bits. TABLE 1 IFW versus LS Code Subset Size Number of IFW LS Codes [0,0] 128 [−1, +1] 64 [−3, +3] 32 [−4, +4] 16

[0058] Signal Transmission

[0059]FIG. 7 illustrates transmissions within each of two cells withinsystem 10 of FIG. 1. The transmission signals include symbols 108, asillustrated in FIG. 2, separated by gaps in time of varying duration. Inthe example illustrated, signals are transmitted to mobile user 24within a first cell that is assigned a first LA code. The first LA codedefines a time boundary sequence. The first LA code defines the chipsizes of gap 1, gap 2, etc. For mobile user 34 within a second cell ofthe first cell's neighborhood, signals are transmitted having a secondLA code defining chip sizes of gap 3, gap 4, etc. The LA sequence foreach mobile user 24, 34 continues with other gap durations betweenmodulated LS codes. The particular LA sequence identifies the basestation from which the transmissions are sent. The gap between symbolsis a time duration during which no information is sent. The sequence ofgaps, or the LA sequence, is effectively a code assigned to a basestation or other transmitting source. The total frame length for each ofmobile user 24 and mobile user 34 is consistent; only the time durationand order of the gaps between modulated LS codes is different amongusers. Each frame complies with a predetermined format and/or protocolthat defines the time allowed for an entire frame. The LA codeeffectively sets the symbol boundaries within a frame. Note that fortransmissions from a given base station, the sub-frame pattern, i.e., LAcode, repeats in successive frames. Alternate embodiments may alter theframe length per user. Note that within a cell and/or sector, a basestation uses a single LA sequence. Neighboring cells will be assigneddifferent LA sequences.

[0060] Within a cell and/or sector each mobile user is assigned a uniqueLS code. However, neighboring cells/sectors may reuse the LS codes, andmost likely will have at least one LS code in common. The common use andreuse of LS codes by neighboring cells and/or sectors createsinterference that may impede correct reception of transmitted signals byone or multiple mobile users in neighboring cells/sectors. The exemplaryembodiment utilizes the IFW characteristics of a LAS-CDMA system byforming subsets of codes based on a desired IFW.

[0061] Assignment of LS Codes

[0062]FIG. 8A illustrates assignment of LS code subsets to neighboringcells within a system 700 having cells 702, 704, 706, 708, 710, 712. Asillustrated, a unique LS code subset is assigned to each neighboringcell. For example, a subset A is assigned to cell 702. The neighbors tocell 702 are each assigned a different and unique subset of LS codes,wherein subset B is assigned to cell 704, subset C is assigned to cell706, subset D is assigned to cell 708, subset E is assigned to cell 710,and subset F is assigned to cell 712. In this way, no mobile user incell 702 will be assigned a code used in a neighboring cell. There is noreuse within the neighboring cells of cell 702.

[0063] Note that not all neighbor cells of cell 702 may be assigneddifferent subsets. One assignment schedule is illustrated in FIG. 8B,wherein no two neighbor cells have a common LS code set assignment. Theillustrated schedule assigns three LS code subsets to the cells withinsystem 700. The subset A is assigned to cell 702. The subset B isassigned to cells 704 and 708, as these two cells are not neighbors.Subset C is assigned to cells 706 and 710. Note that subset A is thenassigned to cell 712, as this cell is not a neighbor of cell 702. As inthe schedule illustrated in FIG. 8A, no neighboring cells are assigned acommon code set.

[0064] In accordance with one embodiment, a process of assigning LS codesubsets determines the size of the IFW. This is the acceptable level ofinterference allowed in the system. For example, an IFW=[0,0] indicatesthat no offsets maintain the desired cross-correlation property. Inother words, there is no window of offsets that is interference free.Similarly, an IFW=[−1,+1] indicates that a group of LS codes can befound that maintains these properties for offsets of +1 to (−1). Thesize of the LS subset is calculated as a function of the IFW selected.Specific code subsets are formed, wherein the number of codes availablefor a subset is determined from the selected IFW and the length of theLS codes. The LS codes may be assigned to subsets sequentially or insome other fashion, wherein in one embodiment, at least three subsetsare formed providing complete isolation of cells with a honeycomblayout, as illustrated in FIGS. 8A and 8B. Alternate embodiments mayincorporate alternate numbers of subsets and allow different isolation.Still other embodiments may incorporate additional subsets to provideadditional isolation of cells. Each system may be implemented toaccommodate the vagaries of the transmission environment. For example,one system may require less isolation due to a sparse concentration ofexpected mobile users or due to a specific type of terrain, while othersystems may require additional isolation beyond neighboring cells due toa dense concentration of users. As an example, consider the caseillustrated in FIG. 6, for a total of 128 LS codes, wherein a desiredIFW=[−1,+1] corresponds to 64 LS codes per subset. The available codesmay then be assigned to subsets.

[0065] In one embodiment the system 10 selects the IFW based on themulti-path propagation delay profile of a mobile user. Note that the MAIand ISI within the IFW may be reduced. It is more difficult to remove orreject the MAI and ISI outside of the window. As the size of the IFWincreases, the number of LS codes that maintain the IFW decreases.However, the number of LS code subsets increases. For example, asdiscussed hereinabove, with an LS code length of 128 bits, anIFW=[−1,+1] results in subset size of 64 LS codes. As there are a totalof 128 codes, this allows two subsets of 64 LS codes. For anIFW=[−3,+3], the resultant LS code subset size is 32, and there are,therefore, four subsets that may be formed from the 128 LS codes.

[0066] A LS code subset is then assigned to one cell. A check is made tosee if the same code subset is used in a neighboring cell. This is toensure that neighboring cells are using distinct subsets. If noneighboring cell is using the same subset, processing continues todetermine if all cells have been assigned. If a neighboring cell usesthe same LS code subset, processing continues to select a different LScode subset. The different LS code subset may be the next sequentialsubset of codes or may be a disjoint set of codes. The processingcontinues until all cells in the system have been assigned code subsets.

[0067] The process will typically be performed by a base stationcontroller (not shown) that provides control for multiple base stationswithin a wireless telecommunication system. The processing mayalternately be performed by each base station, wherein each base stationwithin a system, such as stations 22, 32, 42, 52, 62, 72, communicateswith each other base station in the selection of LS code subsets. Insuch an embodiment, once a base station has selected a LS code subset,this selection is communicated to other base stations within the system.Negotiation between base stations may be used to allow each base stationto isolate communications within the corresponding cell from neighboringcells, i.e., the neighborhood, while allowing each neighbor to performthe same with respect its neighborhood. The assignment of LS codesubsets is a dynamic process and may be modified or changed due tochanging circumstances within a given system. When for example, thedesired IFW changes, it may be necessary to change the LS codeassignments. In this case, fewer codes may be available to form subsets.In another instance, the concentration of mobile users may change,requiring a smaller IFW and making more codes available.

[0068] According to one embodiment, a wireless LAS-CDMA communicationsystem implements multiple LS code subsets, wherein each LS code subsetis assigned to a cell and/or sector within the system and differentsubsets are assigned to neighboring cells. The subsets are formed toachieve a desired IFW, wherein the IFW determines the number of codes ineach subset. Application of such subsets to neighboring cells reducesthe interference between neighboring cells. In one embodiment, the sizeof the subsets is determined by the length of the codes and the IFWaccording to an arborescence structure. In this way, the interferenceexperienced by neighboring cells in a wireless communication system isreduced.

[0069] Thus, a novel and improved method and apparatus identifyingmobile users in a wireless transmission in a wireless communicationsystem has been described. Those of skill in the art would understandthat the data, instructions, commands, information, signals, bits,symbols, and chips that may be referenced throughout the abovedescription are advantageously represented by voltages, currents,electromagnetic waves, magnetic fields or particles, optical fields orparticles, or any combination thereof.

[0070] Those of skill would further appreciate that the variousillustrative logical blocks, modules, circuits, and algorithm stepsdescribed in connection with the embodiments disclosed herein may beimplemented as electronic hardware, computer software, or combinationsof both. The various illustrative components, blocks, modules, circuits,and steps have been described generally in terms of their functionality.Whether the functionality is implemented as hardware or software dependsupon the particular application and design constraints imposed on theoverall system. Skilled artisans recognize the interchangeability ofhardware and software under these circumstances, and how best toimplement the described functionality for each particular application.

[0071] As examples, the various illustrative logical blocks, modules,circuits, and algorithm steps described in connection with theembodiments disclosed herein may be implemented or performed with adigital signal processor (DSP); an application specific integratedcircuit (ASIC); a field programmable gate array (FPGA) or otherprogrammable logic device; discrete gate or transistor logic; discretehardware components such as, e.g., registers and FIFO; a processorexecuting a set of firmware instructions; any conventional programmablesoftware module and a processor; or any combination thereof designed toperform the functions described herein. The processor may advantageouslybe a microprocessor, but in the alternative, the processor may be anyconventional processor, controller, microcontroller, or state machine.The software modules could reside in RAM memory, flash memory, ROMmemory, EPROM memory, EEPROM memory, registers, hard disk, a removabledisk, a CD-ROM, or any other form of storage medium known in the art.The processor may reside in an ASIC (not shown). The ASIC may reside ina telephone (not shown). In the alternative, the processor may reside ina telephone. The processor may be implemented as a combination of a DSPand a microprocessor, or as two microprocessors in conjunction with aDSP core, etc.

[0072] The previous description of the preferred embodiments is providedto enable any person skilled in the art to make or use the presentinvention. The various modifications to these embodiments will bereadily apparent to those skilled in the art, and the generic principlesdefined herein may be applied to other embodiments without the use ofthe inventive faculty. Thus, the present invention is not intended to belimited to the embodiments shown herein but is to be accorded the widestscope consistent with the principles and novel features disclosedherein.

What is claimed is:
 1. An apparatus adapted for a wirelesscommunications supporting Large Area Synchronized-Code Division MultipleAccess (LAS-CDMA) transmissions, the transmissions using LS codes forspread-spectrum modulation, the apparatus comprising: means fordetermining a size of an interference free window (IFW); means forcalculating a plurality of subsets of LS codes, each subset comprising anumber of LS codes as a function of the IFW; means for assigning a firstof the plurality of subsets to a first portion of the system; and meansfor assigning a second of the plurality of subsets to a second portionof the system.
 2. The apparatus of claim 1, wherein the means forcalculating further comprises: means for determining a number of subsetsfor application within the system; means for determining the firstsubset of LS codes having null cross-correlation with respect to eachother; and means for determining the second subset of LS codes havingnull cross-correlation with respect to each other.
 3. The apparatus ofclaim 1, further comprising: means for identifying mobile stationswithin the first portion of the system with LS codes from the first ofthe plurality of subsets; and means for identifying mobile stationswithin the second portion of the system with LS codes from the second ofthe plurality of subsets.
 4. The apparatus of claim 1, wherein across-correlation of the LS codes within the first and second of theplurality of subsets is null within the IFW.
 5. The apparatus of claim1, wherein the size of the IFW corresponds to an LS code length, andwherein means for calculating the plurality of subsets furthercomprises: means for generating seed pairs given as: (C1; S1); and (C2;S2); and means for generating a plurality of LS codes of the LS codelength by application of a formula given as: (C1   C2; S1   S2) (C1 −C2;S1 −S2) (C2   C1; S2   S1) (C2 −C1; S2 −S1),

wherein a negative indicates a binary complement of an original element.6. The apparatus as in claim 5, further wherein the number of subsets isat least three.
 7. An apparatus adapted for use in a Large AreaSynchronized-Code Division Multiple Access wireless communicationsystem, the apparatus comprising: means for transmitting a firstcommunication within a first cell, the first communication identifyingat least one mobile station within the first cell by a first LS codewithin a first subset of LS codes; and means for transmitting a secondcommunication within a second cell, the second communication identifyingat least one mobile station within the second cell by a second LS codewithin a second subset of LS codes; wherein a cross-correlation betweenany two LS codes within the first is null within an interference freewindow, and the cross-correlation between any two LS codes within thesecond subset is null within the interference free window.
 8. Theapparatus of claim 7, wherein the first and second subsets of LS codesare part of a set of LS codes defined by the interference free window.9. The apparatus of claim 8, wherein for the set of LS codes comprises128 codes, the interference free window equal to [−1,+1] corresponds to64 available codes for forming subsets.
 10. The apparatus of claim 9,wherein a correspondence between the interference free window and anumber of available codes for forming subsets is based on anarborescence structure.